Spread-spectrum clock generator (SSCG) is used in synchronous digital systems in order to reduce spectral density of the electromagnetic interference (EMI). A synchronous digital system is one that is driven by a clock signal and, because of its periodic nature, has a narrow frequency spectrum. Spread-spectrum clocking reduces the peak radiated energy and electromagnetic emissions.
A SSCG generates a clock signal with a frequency that is intentionally swept (frequency modulated) within a certain frequency range, with a predefined modulation profile. Spread-spectrum clocking does not reduce the total power of the harmonic noise but it spreads the energy of each clock harmonic evenly over a dedicated bandwidth. In this way, the peak power level at each clock harmonic is reduced as much as 10-20 DB, depending on the modulation depth and the modulation profile.
A SSCG typically spreads energy over a larger bandwidth thereby effectively reduces electrical and magnetic readings within narrow bandwidths. Distributing this same energy into a larger bandwidth prevents systems from putting enough energy into any one narrowband to exceed the statutory limits.
However, spread-spectrum clocking, like other kinds of dynamic frequency changes, may create challenges such as clock/data misalignment or clock skew.